Transceivers having a single antenna for transmitting and receiving signals typically use an antenna switch to switch the antenna between the transmitter and receiver. Typical antenna switches connect either the transmitter or receiver to the antenna while also isolating the transmitter and receiver from each other. Generally, the loss caused by the switch and the isolation provided by the switch drives the antenna switch design. Other parameters, e.g., power consumption, power handling capability, and/or linearity, may also be considered.
In addition to isolating the transmitter and receiver from each other, an antenna switch may further isolate the transmitter and/or receiver from undesirable power surges that may damage the transmitter and/or receiver, e.g., power surges caused by electrostatic discharge (ESD). When a transceiver chip includes separate transmit and receive pins, a clamp at the receive pin input protects the receiver from power surges. A clamp may also be used to protect the transmitter. For example, when the transmitter uses a pull-up inductor, a clamp may be placed at the supply side of the inductor, not at the transmit-side, so that the inductor acts as a short for power surges. Because the transmitter comprises a large device, it generally will survive any power surge signals applied at the transmit side of the transmitter.
While a clamp provides good surge protection for transceivers having separate transmit and receive pins on the IC chip, such clamps do not provide a good power surge solution when the transceiver combines the transmit and receive pins at one pin on the chip. In this scenario, a clamp applied to the receiver input will limit the transmit output power; the clamp will simply rectify the RF levels. Thus, an alternate solution is desired.
The size of the antenna switch also represents another design consideration, especially given that integrating a switch in sub-micron CMOS processes has become quite popular. The low supply voltage associated with such CMOS solutions, however, may be problematic. A peak transmission voltage associated with a high transmission power that exceeds the switch supply voltage limits the functionality of the switch. For example, when the transmit power level exceeds the switch supply voltage, parasitic devices present in the switch start conducing, which causes strong non-linearity, high losses, and compression effects.
One way to address this problem comprises connecting the switch to a ¼ wave length stub, or equivalently to a resonant LC tank coil. Such solutions have low loss, (less than 1 dB), provide good isolation (more than 20 dB), provide good linearity, and provide good power handling. This solution, however, requires an undesirably large area inductor. Both the ¼ wavelength stub and the equivalent LC tank coil are generally large, particularly relative to the other components on the active area of the chip. As chip components continue to decrease in size, the size of the active area likewise continues to decrease, which makes the size of the stub/coil even more dominant. At some point, ¼ the wavelength stub/LC tank coil dominates the chip size so much that further attempts at reducing the size of the chip have diminishing returns.
While various other techniques may be used to avoid the problems associated with conventional antenna switches integrated via sub-micron CMOS processes, such techniques generally require additional voltages and/or special layout structures with switches in a biased well. The additional voltages may undesirably increase the power consumption of the device, while biased well switches may undesirably cause latch-up.
In view of the problems with conventional antenna switches, there remains a need for alternative antenna switches that do not overly increase the size of a transceiver chip layout while still providing low loss, good isolation, good linearity, good power surge protection, etc.